Koki Hibi

Investigation of neutron reaction behavior in water-cooled FBR with MOX fuel

A water-cooled FBR with MOX fuel is under development using current LWR technologies and its design targets are to achieve a BR value, which is a ratio of a fissile plutonium content in a spent fuel to one in a fresh fuel, larger than 1.0 and to obtain a negative void reactivity. This study has been performed to clarify the reasons why the water-cooled FBR has high sensitivities for core specifications. The water-cooled FBR must reduce an X/HM, which is a ratio of hydrogen or deuteron to heavy metal atomic number densities in a unit core volume, to about 0.2 for light water cooling case and 1.0 for heavy water case to attain the required breeding performance. Although the water-cooled FBR is a fast reactor, typical neutron energy values are about 0.4 ke V for the former case and about 13ke V for the latter case, and such relatively soft neutron spectra compared with the sodium-cooled FBR one give significant differences on nuclear performance. The water-cooled FBR has a positive void reactivity due to the dynamic reaction rate spectrum changes from normal to voided conditions, named neutron spectrum shift effect. The spectrum shift gives a negative void reactivity in depleted UO2 fuel, so the optimization of the fuel arrangements among the MOX fuel reduces the void reactivity.

Study on bubbly flow behavior in natural circulation reactor by thermal-hydraulic simulation tests with SF6-Gas and ethanol liquid

An advanced experimental technique has been developed to simulate two-phase flow behavior in a light water reactor (LWR). The technique applies three kinds of methods; (1) use of sulfur-hexafluoride (SF6) gas and ethanol (C2H5OH) liquid at atmospheric temperature and a pressure less than 1.0MPa, where the fluid properties are similar to steam-water ones in the LWR, (2) generation of bubble with a sintering tube, which simulates bubble generation on heated surface in the LWR, (3) measurement of detailed bubble distribution data with a bi-optical probe (BOP), (4) and measurement of liquid velocities with the tracer liquid. This experimental technique provides easy visualization of flows by using a large scale experimental apparatus, which gives three-dimensional flows, and measurement of detailed spatial distributions of two-phase flow. With this technique, we have carried out experiments simulating two-phase flow behavior in a single-channel geometry, a multi-rod-bundle one, and a horizontal-tube-bundle one on a typical natural circulation reactor system. Those experiments have clarified a) a flow regime map in a rod bundle on the transient region between bubbly and churn flow, b) three-dimensional flow behaviour in rod-bundles where inter-subassembly cross-flow occurs, c) bubble-separation behavior with consideration of reactor internal structures. The data have given analysis models for the natural circulation reactor design with good extrapolation.

Design of Recycle Pressurized Water Reactor with Heavy Water Moderation

Abstract This study presents the conceptual design of the recycle pressurized water reactor (RPWR), which is an innovative PWR fueled with mixed oxide, moderated by heavy water, and having breeding ratios around 1.1. Most of the systems of RPWR can employ those of PWRs. The RPWR has no boric acid systems and has a small tritium removal system. The construction and operation costs would be similar to those of current PWRs. Heavy water cost has decreased drastically with up-to-date producing methods. The reliability of the systems of the RPWR is high, and the research and development cost for RPWR is very low because the core design is fundamentally based on the current PWR technology.